Vehicle engine systems may utilize an external exhaust gas recirculation (EGR) system to reduce NOx emissions and increase engine efficiency. For example, the external EGR system may couple an engine exhaust manifold to an engine intake manifold via an EGR passage. An EGR valve disposed within the EGR passage may be controlled to achieve a desired intake air dilution for the given engine operating conditions (e.g., engine speed, engine load, and engine temperature) to maintain desirable combustion stability while providing emissions and fuel economy benefits. Degradation of the EGR system may degrade engine performance, for example, by increasing emissions or reducing fuel economy. For example, if the EGR valve becomes degraded and stuck open, such as due to carbon deposits blocking valve closure, EGR may be delivered when no EGR is desired. This may lead to rough idle, stalling, and an increased occurrence of misfire. Therefore, various diagnostic procedures may be periodically or opportunistically performed to monitor operation of the EGR system, including the EGR valve.
Other attempts to identify degradation of the EGR system include performing a diagnostic routine while the vehicle is operating in a coastdown mode, while an accelerator pedal is not depressed and the vehicle is moving. One example approach is shown by Whitney et al. in U.S. Pat. No. 8,316,828. Therein, when the vehicle is in the coastdown mode, the EGR valve is opened while a throttle area is maintained. An error of the EGR system is selectively diagnosed based on a pressure increase measured in an intake manifold of the vehicle when the EGR valve is opened, such as when the measured pressure increase is less than a minimum pressure increase.
However, the inventor herein has recognized that hybrid vehicles and stop/start vehicles, in which the engine is shut off (e.g., pulled down) while the vehicle is in motion, provide an additional opportunity to check whether the EGR valve is stuck open, such as due to an obstruction preventing the valve from fully closing. This may be particularly beneficial because hybrid vehicles commonly include an open-loop stepper motor for adjusting EGR valve position, and thus, there is no direct feedback on the EGR valve position. Because the engine pull-down occurs as a part of hybrid and stop/start vehicle operation, no additional throttle position and EGR valve position adjustments are needed, and monitoring the EGR valve during the engine pull-down is non-intrusive.
In one example, the issues described above may be addressed by a method, comprising: propelling a vehicle by an engine having a throttle coupled to an intake manifold; recirculating a portion of engine exhaust to the intake manifold through an exhaust gas recirculation (EGR) valve; and checking operation of the EGR valve, based on both intake manifold pressure and exhaust pressure, after closing the throttle and the EGR valve when propelling the vehicle with an electric motor while the engine spins down to rest. In this way, an EGR valve that is stuck open, enabling undesired recirculation of exhaust gas, may be identified.
As one example, checking operation of the EGR valve may include comparing the intake manifold pressure to the exhaust pressure, such as a difference or a ratio of the intake manifold pressure and the exhaust pressure, while the engine spins down to rest. It may be indicated that the EGR valve is degraded and stuck open in response to the difference not reaching or exceeding a threshold difference or in response to the ratio not reaching or decreasing below a threshold ratio. In this way, the EGR valve may be identified as stuck open without a position sensor and without altering throttle and EGR valve control. Further, engine operating parameters may be adjusted in order to compensate for undesired EGR flow, such as by advancing spark timing and decreasing fueling. Thereby, emissions and degraded engine performance, such as due to misfire, rough idle, and stalling, may be reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.